(emacs)Undo

Undoing Changes
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Emacs allows all changes made in the text of a buffer to be undone,
up to a certain amount of change. Each buffer records changes
individually, and the undo command always applies to the current
buffer. Usually each editing command makes a separate entry in the undo
records, but some commands such as `query-replace' make many entries,
and very simple commands such as self-inserting characters are often
grouped to make undoing less tedious.
`C-x u'
Undo one batch of changes--usually, one command worth (`undo').
`C-_'
The same.
The command `C-x u' or `C-_' is how you undo. The first time you
give this command, it undoes the last change. Point moves back to
where it was before the command that made the change.
Consecutive repetitions of the `C-_' or `C-x u' commands undo
earlier and earlier changes, back to the limit of what has been
recorded. If all recorded changes have already been undone, the undo
command prints an error message and does nothing.
Any command other than an undo command breaks the sequence of undo
commands. Starting at this moment, the previous undo commands are
considered ordinary changes that can themselves be undone. Thus, you
can redo changes you have undone by typing `C-f' or any other command
that will have no important effect, and then using more undo commands.
If you notice that a buffer has been modified accidentally, the
easiest way to recover is to type `C-_' repeatedly until the stars
disappear from the front of the mode line. At this time, all the
modifications you made have been cancelled. If you do not remember
whether you changed the buffer deliberately, type `C-_' once, and when
you see the last change you made undone, you will remember why you made
it. If it was an accident, leave it undone. If it was deliberate,
redo the change as described in the preceding paragraph.
Whenever an undo command makes the stars disappear from the mode
line, it means that the buffer contents are the same as they were when
the file was last read in or saved.
Not all buffers record undo information. Buffers whose names start
with spaces don't; these buffers are used internally by Emacs and its
extensions to hold text that users don't normally look at or edit.
You cannot undo mere cursor motion; only changes in the buffer
contents save undo information. However, some cursor motion commands
set the mark, so if you use these commands from time to time, you can
move back to the neighborhoods you have moved through by popping the
mark ring (Note:Mark Ring.).
When the undo information of a buffer becomes too large, Emacs
discards the oldest undo information from time to time (during garbage
collection). You can specify how much undo information to keep by
setting two variables: `undo-limit' and `undo-strong-limit'. Their
values are expressed in units of bytes of space.
The variable `undo-limit' sets a soft limit: Emacs keeps undo data
for enough commands to reach this size, and perhaps exceed it, but does
not keep data for any earlier commands beyond that. Its default value
is 20000. The variable `undo-strong-limit' sets a harsher limit: the
command which pushes the size past this amount is itself forgotten.
Its default value is 30000.
Regardless of the values of those variables, the most recent change
is never discarded, so there is no danger that garbage collection
occurring right after an unintentional large change might prevent you
from undoing it.
The reason the `undo' command has two keys, `C-x u' and `C-_', set
up to run it is that it is worthy of a single-character key, but on
some keyboards it is not obvious how to type `C-_'. `C-x u' is an
alternative you can type in the same fashion on any terminal.